FIG. 12 is a perspective view of a hydraulic excavator shown as an example of construction machinery. In this hydraulic excavator, a swing upperstructure 21 is arranged on a travel base 20, and this swing upperstructure 21 is provided with a swing frame 22 to which the present invention is applied.
As a conventional structure of the swing frame 22, one illustrated in FIGS. 13 and 14 has been proposed, for example. FIG. 13 is a plan view, while FIG. 14 is a side view. The conventional swing frame structure shown in these FIGS. 13 and 14 is provided at a front position thereof with a center frame 23 and at a rear position with a tail frame 24. The center frame 23 is provided with a bottom plate 25, a pair of webs, in other words, side plates 26,27 arranged upright on the bottom plate 25 and joined to the bottom plate 25 by welding, and a plate member, in other words, a reinforcement plate 28 joined by welding between these side plates 26 and 27.
In a conventional technology with the center frame 23 constructed as mentioned above, the fabrication of the center frame 23 generally requires a large jig to fixedly hold the paired side plates 26,27 in addition to a jig for fixedly holding these side plates 26,27 and the reinforcement plate 28. In the state that the paired side plates 26,27 and the reinforcement plate 28 are held by the jig as mentioned above, the paired side plates 26,27 tend to move so that as mentioned above, an additional jig is required to position and fixed hold these side plates 26,27 and the bottom plate 25 relative to each other. In the state that the side plates 26,27, the reinforcement plate 28 and the bottom plate 25 are fixedly held and positioned by the respective jigs, they are subjected to tack welding, and subsequent to final positional adjustments, these side plates 26,27, reinforcement plate 28 and bottom plate 25 are subjected to final welding to integrate them together so that the center frame 23 can be completed.
The tail frame 24, on the other hand, is provided with a pair of frame members 29,30. Lateral beams 31,32 are arranged between these frame members 29 and 30, and are joined to these frame members 29,30. The frame members 29,30 are provided with side plates 29a,30a, to which the lateral beams 31,32 are joined, and upper flanges 29b,30b located on these side plates 29a,30a, respectively. On the lateral beams 32,32, discrete brackets can be mounted to permit the fixed installation of an engine.
FIG. 15 diagrammatically illustrates a connection structure between an upper flange of an I-beam, in other words, a frame member, which constitutes the tail frame arranged in the swing frame structure depicted in FIG. 13, and the side plate which constitutes the center frame, in which FIG. 15(a) is a fragmentary plan view while FIG. 15(b) is a fragmentary side view. It has been a common conventional practice, for example, that a front end face of the upper flange 30b of the frame member 30, which constitutes the tail frame 24, and the side plate 27, which constitutes the center frame 23, are joined together by a weld portion 33 as shown in FIGS. 15(a) and 15(b). As illustrated in FIG. 15(a), this weld portion 33 is formed between abutting portions of the front end portion of the upper flange 30b an the side plate 27, and as depicted in FIG. 15(b), is also formed between the front end portion of the upper flange 30b and a side wall of the side plate 27. This applies equally to the unillustrated joint portion between the upper flange 29b and the side plate 26 which constitutes the center frame 23. It is to be noted that L1 shown in FIG. 15(a) indicates the dimension of a bead extension set to assure sufficient joint strength between the flame member 30 and the side plate 27 of the center frame 23. In particular, the weld portions 33 formed on the side walls of the side plates 26,27 of the center frame 23 are generally formed by manually performing clad welding many times and then applying grinder finishing into tapered shapes while following the shapes of the upper flanges 29b,30b. It is to reduce the concentration of a stress at the weld portions 33, in other words, the welded and joined portions that the grinder finishing is performed into the tapered shape conforming with the shapes of the upper flanges 29b,30b (see, for example, Patent Document 1-JP-B-2719469).
As another conventional technology, there is a swing frame structure that engine-fixing brackets are formed integrally with the above-mentioned lateral beams 31,32 to provide frame-side brackets, namely, engine brackets. The engine brackets constructed as mentioned above are each arranged between a pair of frames, specifically I-beams and joined to these frame members by welding (see, for example, Patent Document 2-JP-A-2000-064353).
The above-mentioned conventional technology disclosed in Patent Document 2 has a merit that it requires a smaller number of members than the conventional technology disclosed in Patent Document 1, but upon fabrication of the tail frame 24, it requires a jig for fixing the paired I-beams and the engine brackets in a state that the engine brackets are positioned between these I-beams. As described above, in the state that the I-beams and the engine brackets are fixedly held and positioned by a jig, the side walls of the I-beams and the engine brackets are subjected to tack welding, and by final welding, these I-beams and engine brackets are integrated together to complete a tail frame.